3.2 Electrophoretic deposition (EPD)
3.2.1 EPD in polar solvents
Polar solvents are commonly employed as the particle suspension media for EPD technique. Typically, the particles suspended in such solvents acquire the surface charge easily, which is important for EPD. Common polar solvents used for EPD are water (H2O),
ethanol/ethanol-water mixtures.51 EPDs, performed in polar solvents, are further classified in the two types: aqueous EPD and non-aqueous EPD.86, 87 In aqueous EPD, water is used as the particle suspension medium. Since water is non-toxic, non-flammable, and abundant, this technique is environmentally safe and cost-effective. However, this technique is suscep- tible to electrochemical reactions at the electrodes, which affects the quality of the EPD film.88 Even at low applied voltages, aqueous suspensions of particles conduct high cur- rents due to high electrolytic currents, passing through water. If the electric fields are high enough, electrolysis of water occurs, and gas bubbles (H2 & O2) are formed at the electrodes, which affects the homogeneity of the deposited films. Hence, the range of operating volt- ages for aqueous EPD is very limited. In non-aqueous EPD, organic solvents are employed as the particle suspension media. Generally, the possibility of electrolysis is significantly minimized or completely absent in these solvents because of their low dielectric constants and the minimal presence of water combined. Also, these solvents have higher dielectric breakdown potentials than that of water, which facilitate the larger operating voltage range.
In addition, low conductivity and good chemical stability are typical characteristics of these solvents. However, toxicity, flammability, and high costs are among the drawbacks of these liquids. Both aqueous and non-aqueous EPDs are employed commercially.51
Irrespective of the type of solvent used (aqueous or non-aqueous) for EPD, the ability to acquire charge on the surface of particles, suspended in the solvent, is essential for deposition. When particles are suspended in a polar solvent, usually a charge develops at the solid-liquid interface.89 Mechanisms for the development of charge on particles in water are well understood, which include adsorption of polar molecules at the interface, selective adsorption of free ions from liquid onto the particle surface, and dissociation of solid ions from the surface of particles into the solvent.51, 52 Protons have been identified as the charge determining ions in aqueous suspensions of ceramic particles, specifically oxides.90 Thus, the surface charge of the particles can be modified by controlling pH of an aqueous EPD suspension.
Unlike aqueous EPD suspensions, knowledge of the mechanisms that are responsible for the development of surface charge in non-aqueous EPD suspensions is limited. The concept of hydrogen concentration (pH) as a measure of acidity or alkalinity looses its validity in non-aqueous solvents because of the absence of hydrogen ions. Wang et al observed that alumina particles acquire the surface charge in ethanol by addition of acetic acid or tetra-methyl-ammonium-hydroxide.91 The surface charge characteristics of alumina particles suspended in ethanol were similar to that of the oxide particles suspended in water.92 Vandeperre et al performed potentiometric titration experiments to determine the charging of various ceramic particles (oxides, borides, carbides, nitrides), and the charging characteristics were compared to the sign of the electrophoretic mobility measured in acidic and alkaline non-aqueous solvents.93 These experiments confirmed that the charging of ceramic particles in a non-aqueous medium is analogous to charging in water. The small amount of residual water in non-aqueous solvents has been suggested to play a role in charging of yttria-stabilized zirconia (YSZ) particles.94 The addition of iodine, a charging agent, in acetone or acetylacetone facilitates the formation of protons, which are adsorbed onto the surface of suspended particles.95, 96 Thus, the ceramic research community has acquired a working knowledge of the control of the surface charge of particles suspended in non-aqueous solvents.
Particles with surface charge, suspended in liquid media, experience interparticle forces, such as the van der Waals attractive force, electrostatic repulsive force, and steric repulsive force. Stability of the particle suspension is governed by the net interparticle forces. Well-stabilized, non-agglomerated particle suspensions are necessary for EPD. The repulsive forces between the particles should exceed the van der Waals attractive forces to achieve well-stabilized particle suspension. The classical DLVO theory, developed by Derjaguin and Landau97 and Verwey and Overbeek,98 describes the relationship between the interparticle forces and energies of interaction to stabilize the suspensions. This theory considered only the electrostatic and van der Waals forces. The DLVO theory was primarily
Figure 3.2: Schematic of electrostatically stabilized particles in suspension. Electrostatic repulsive forces between the particles supersede van der Waals attraction forces to obtain well-stabilized suspension.
developed for the electrostatically stabilized suspensions, i.e particle suspensions in polar media. Figure 3.2 shows schematic of the electrostatically stabilized particles.
In a well-stabilized particle suspension, the charged particles move with a velocity under the influence of an electric field, a phenomenon which was first studied by Smolu- chowski99 The mobility (μ) of the particles under electrophoretic forces, known as elec- trophoretic mobility, is related to the zeta potential of the particle (ζ), the solvent viscosity (η), relative permittivity of the solvent ( r), and the permittivity of vacuum ( 0) through the H¨uckel equation (Equation 3.3).
μ= 2· 0· r·ζ
3·η (3.3)
Typically, electrophoretic mobility of the particles is measured by dynamic light scattering (DLS) experiments. The particles can have positive or negative electrophoretic mobility based on their zeta potential. Negatively charged particles have negative electrophoretic mobility, while positively charged particles have positive electrophoretic mobility. Thus, the sign of electrophoretic mobility indicates where the particles deposit (anode/cathode).
Typically, all the particles suspended in polar solvents have one type of electrophoretic mobility (positive or negative). Hence, the particles deposit only on one electrode (e.g.
cathode EPD and anodic EPD).
Recently, carbon nanotubes (CNTs) have been deposited successfully with aqueous and non-aqueous EPD technique.77 Typically, CNTs require a post-synthesis treatment to remove impurities and to isolate individual tubes from their aggregates.100–102 The CNTs are functionalized with acidic surface groups, which are developed during the post synthesis purification treatment.101, 102 These acidic group electrostatically stabilize the CNTs in water or other non-aqueous polar solvents, by developing a negative surface charge. The resulting repulsion between the CNTs suspended in the solvent forms a well-stabilized CNT suspension. Similar to the EPD of ceramics, the CNTs have been deposited with different polar solvents.77 EPD of the CNTs explored in this dissertation involved suspension of the CNTs in water.[Section 6.2.2]